// split featureCollection df
var nSize = df.size().getInfo()
for (var ii=0; ii<nSize;ii=ii+nChunk){
    // var ii = 0
    var vFeatures_sub = ee.FeatureCollection(df.toList(nChunk,ii))
    // do some operation on subset of df
}


// visualization
// plot the polygon regions on the screen, with no fillcolor, and black edge
Map.addLayer(
    regions.style({color:'black', fillColor: '00000000'}),
    {},
    "region-13"
    )